As outlined in my comment, I hypothesise it must have been the growth of exothermic bacteria. As the name suggests, these produce heat. An especially interesting phenomenon concerns the danger of fire erupting from moist hay, as the bacteria feed from it and dry it simultaneously! This is outlined here. A memorable quote from that article:
Dry hay (stored at 15 percent moisture or less) is safe for long-term storage. However, if the hay has become wet the quality has been permanently changed and the potential fire hazard from spontaneous combustion increased.
The same principle underlies the heating of compost, about which you can read in greater detail here.
Most astounding I found the post dealing with the possible contribution of exothermic gut bacteria to our thermoregulation.
As of now, I don't know about the basic mechanisms by which these bacteria generate heat, but the abstract of this article implies that heat absorbing bacterial growth is extraordinary.
I myself have forgotten everything about chemistry, but from the comments below I gathered that what happens is similar to the oxidations of sugar. You can read examples here, which demonstrate that these are exothermic.